Recall from Section 3.4 that
the PostgreSQL server provides a large number of run-time
configuration variables. You can set database-specific default
values for many of these settings.

For example, if for some reason you want to disable the
GEQO optimizer for a given
database, you'd ordinarily have to either disable it for all
databases or make sure that every connecting client is careful to
issue SET geqo TO off;. To make this
setting the default you can execute the command

ALTER DATABASE mydb SET geqo TO off;

This will save the setting (but not set it immediately) and in
subsequent connections it will appear as though SET geqo TO off; had been called right before the
session started. Note that users can still alter this setting
during the session; it will only be the default. To undo any such
setting, use ALTER DATABASE dbname RESET varname;.